The present invention relates to polyurethane dispersions, a process for their production and solvent-free primers for substrates containing these dispersions. The invention also relates to substrates, in particular plastics, primed with the dispersions, which have improved adhesion and higher gloss.
Paint on high-grade plastic parts is applied in three layers. Such a layered structure comprises a primer, base coat and clear coat. The primer acts as the bonding agent between substrate and paint. An important property of a primer should therefore be to ensure good bonding on different plastic substrates. Aqueous primers can sometimes not adequately fulfil this basic requirement.
A further property of the primer is the possibility of covering and/or levelling defects in the surface of the molding. Since this cannot be entirely successful for all defects, these defects are conventionally eliminated after the priming step by regrinding. Good grinding ability on the part of the primer is therefore essential. Surface defects that are not eliminated at this point in the manufacturing process must be reworked at a later stage of the production process, involving considerably greater effort and higher costs. Otherwise the molding would be unable to be used. The discovery of even the smallest surface defect is therefore of enormous significance. Since defects and irregularities are easier to detect on glossy surfaces, a good primer should display a minimum gloss.
The requirements of a primer as described above are already met by solvent based (organically dissolved) polyurethanes of the prior art. However, these organically dissolved primers contribute to a considerable extent to the overall solvent emissions of a painting line. The use of organically dissolved primers in modern painting lines is therefore not desirable in ecological and economic terms. The lowest possible contribution to the volatile organic content (VOC) is a further requirement of a suitable primer. There is thus an urgent need for solvent-free aqueous primers for plastic parts that satisfy the requirements described above.
As yet, however, no systems based on purely aqueous polyurethane dispersions are known that satisfactorily solve all of the aspects of a primer as described above.
U.S. Pat. No. 5,548,016 describes coatings containing mixtures of aqueous preparations of polyurethanes and polyacrylates. These are suitable as a primer for the painting of plastic parts if 10 to 25 wt. % of an organic, water-miscible solvent (co-solvent) are added.
DE-A 2 651 506 describes a process for the production of polyurethanes that are dispersible in water. The process products disclosed therein are not suitable as a primer in the painting process for plastic parts, however, as they do not have the necessary adhesive properties.
It is an object of the present invention to provide a solvent-free, aqueous primer for substrate surfaces, in particular plastics, that meet the requirements described above.
It has now been found that coating compositions based on the polyurethane dispersions according to the invention are outstandingly suitable as a primer for the painting of diverse substrate surfaces, preferably plastic substrates.
The present invention relates to an aqueous polyurethane resin dispersion containing
i) a polyol component with a number-average molecular weight of at least 300 daltons, containing at least one polyether diol initiated on an aromatic diol,
ii) a polyol component with a number-average molecular weight of 62 to 299 daltons,
iii) a monofunctional isocyanate-reactive compound with an ethylene oxide content of at least 50 wt. % and a molecular weight of at least 400 daltons,
iv) a polyisocyanate,
v) an aliphatic polyamine with a molecular weight of 60 to 300 daltons or hydrazine, and
vi) a hydrophilic aliphatic diamine.
The invention also relates to a coating composition containing
A) 30 to 90 parts by weight of this polyurethane resin dispersion,
B) 5 to 60 parts by weight of an inorganic filler,
C) 1 to 60 parts by weight of a water-miscible polyisocyanate,
D) 0.1 to 30 parts by weight of a pigment, and
E) 1 to 15 parts by weight of a paint additive,
whereby the sum of the components is 100 parts by weight.
The invention also relates to a process for production of the dispersions and a substrate primed with the dispersion.
The coating compositions containing the polyurethane resin dispersions according to the invention show outstanding adhesion both to diverse substrate surfaces, in particular plastic substrates, and to subsequent paint films, improved resistance to condensation and solvents in the overall paint structure and an extremely low VOC.
In the context of the invention the term xe2x80x9cpolyurethanexe2x80x9d also includes xe2x80x9cpolyurethane polyureasxe2x80x9d, i.e., high-molecular compounds containing urea groups in addition to urethane groups.
Suitable structural components i) include organic compounds containing at least two free hydroxyl groups, which are capable of reacting with isocyanate groups. Examples of such organic compounds include higher-molecular compounds from the classes of polyester, polyester amide, polycarbonate, polyacetal and polyether polyols with a number average molecular weight of at least 300, preferably 500 to 8000, particularly preferably 800 to 5000 daltons. Preferred compounds are, for example, those containing two hydroxyl groups (difunctionally), such as polyester diols or polycarbonate diols.
Examples of polyester polyols include linear polyester diols or weakly branched polyester polyols, prepared from aliphatic, cycloaliphatic or aromatic dicarboxylic or polycarboxylic acids or anhydrides thereof, such as succinic, glutaric, adipic, pimelic, suberic, azelaic, sebacic, nonane dicarboxylic, decane dicarboxylic, terephthalic, isophthalic, o-phthalic, tetrahydrophthalic, hexahydrophthalic or trimellitic acid, and acid anhydrides, such as o-phthalic, trimellitic or succinic anhydride or a mixture thereof with polyhydric alcohols, such as, e.g., ethanediol, diethylene, triethylene, tetraethylene glycol, 1,2-propanediol, dipropylene, tripropylene, tetrapropylene glycol, 1,3-propanediol, butane-1,4-diol, butane-1,3-diol, butane-2,3-diol, pentane-1,5-diol, hexane-1,6-diol, 2,2-dimethyl-1,3-propanediol, 1,4-dihydroxycyclohexane, 1,4-dimethylol cyclohexane, octane-1,8-diol, decane-1,10-diol, dodecane-1,12-diol or mixtures thereof, optionally with the additional use of higher-functional polyols, such as trimethylol propane or glycerol. Examples of polyhydric alcohols for production of the polyester polyols also include cycloaliphatic and/or aromatic dihydroxyl and polyhydroxyl compounds. Instead of the free polycarboxylic acid the corresponding polycarboxylic anhydrides or corresponding polycarboxylic acid esters of low alcohols or mixtures thereof can also be used to produce the polyesters.
The polyester polyols can also be homopolymers or copolymers of lactones, which are preferably obtained by reacting lactones or lactone mixtures, such as butyrolactone, xcex5-caprolactone and/or methyl xcex5-caprolactone with suitable difunctional and/or higher functional initiator molecules, such as the low-molecular, polyhydric alcohols mentioned above.
Polycarbonates having hydroxyl groups are also suitable as polyhydroxyl components, and include those that can be produced by reacting diols such as 1,4-butanediol and/or 1,6-hexanediol with diaryl carbonates, e.g., diphenyl carbonate, dialkyl carbonate, such as dimethyl carbonate or phosgene, with a number-average molecular weight of 800 to 5000 daltons.
Preferred structural components i) are polyester diols based on adipic acid and glycols such as 1,4-butanediol, 1,6-hexanediol and/or 2,2-dimethyl-1,3-propanediol (neopentyl glycol). Likewise preferred are copolymers of 1,6-hexanediol with xcex5-caprolactane and diphenyl carbonate with a number-average molecular weight of 1000 to 4000 daltons, and 1,6-hexanediol polycarbonate diols with a number-average molecular weight of 1000 to 3000 daltons.
Examples of polyether polyols include the polyaddition products of styrene oxides, of ethylene oxide, propylene oxide, tetrahydrofuran, butylene oxide, epichlorohydrin, and their co-addition and graft products, as well as the polyether polyols obtained by condensation of polyhydric alcohols or mixtures thereof and by alkoxylation of polyhydric alcohols, amines and aminoalcohols.
The structural component i) contains polyether diols initiated on aromatic diols, which are produced, for example, by polyaddition of alkylene oxides, such as propylene oxide, ethylene oxide, butylene oxide or styrene oxide to aromatic diols. Preferred alkylene oxides are propylene oxide and ethylene oxide, propylene oxide is particularly preferred. Examples of suitable aromatic diols include hydroquinone, resorcinol, catechol or 2,2-bis(4-hydroxyphenyl)propane (bisphenol A). Aromatic polycarboxylic acids, such as, e.g., o-, iso- or terephthalic acid can also be used as initiators for the alkoxylation reaction. 2,2-bis(4-hydroxyphenyl)propane is preferred.
Preferred polyether polyols initiated on aromatic diols are the propoxylation products of 2,2-bis(4-hydroxyphenyl)propane (bisphenol A) in the molecular weight range between 300 and 3000 dalton, particularly preferably between 500 and 1250 dalton. The amount of polyether diols initiated on aromatic diols in the structural component i) in the polyurethane dispersions according to the invention is between 3 and 100 parts by weight, preferably between 5 and 50 parts by weight, particularly preferably between 5.5 and 30 parts by weight.
Suitable structural components ii) include diols in the molecular weight range 62 to 299. They include, for example, the polyhydric, in particular, dihydric, alcohols mentioned for the production of the polyester polyols, as well as low-molecular polyester diols, such as, e.g., adipic acid bis(hydroxyethyl)ester. Preferred structural components ii) are 1,2-ethanediol, 1,4-butanediol, 1,6-hexanediol and 2,2-dimethyl propane-1,3-diol. 1,4-butanediol and 1,6-hexanediol are more preferred.
The polyurethane resin dispersions according to the invention display, relative to solids, a content of 1 to 4 wt. % of terminally and/or laterally incorporated ethylene oxide units, which can easily be incorporated by the inclusion of structural components iii) in the isocyanate polyaddition process.
Hydrophilic structural components iii) for the incorporation of terminal chains having hydrophilic ethylene oxide units include compounds having formula (I),
H-Yxe2x80x2-X-Y-Rxe2x80x83xe2x80x83(I)
in which
R represents a monovalent hydrocarbon radical with 1 to 12 carbon atoms, preferably an unsubstituted alkyl radical with 1 to 4 carbon atoms,
X represents a polyalkylene oxide chain with 5 to 90, preferably 20 to 70, chain segments, which consist of at least 40%, preferably at least 65% ethylene oxide units and which in addition to ethylene oxide units can contain of propylene oxide, butylene oxide or styrene oxide units, whereby of the last-named units propylene oxide units are preferred, and
Y and Yxe2x80x2 preferably independently represent oxygen or xe2x80x94NRxe2x80x2xe2x80x94, where Rxe2x80x2 represents R or hydrogen.
Monofunctional polyethers are preferably only used in molar quantities of xe2x89xa610%, relative to the polyisocyanate used to ensure the desired high-molecular structure of the polyurethane elastomers. If larger molar quantities of monofunctional alkylene oxide polyethers are used, the additional use of trifunctional compounds displaying isocyanate-reactive hydrogen atoms is advantageous, with the proviso, however, that the average functionality of the starting compounds is not greater than 2.1. The monofunctional hydrophilic structural components can be produced by analogy to those in DE-OS 2 314 512 or 2 314 513 or in U.S. Pat. Nos. 3,905,929 or 3,920,598 by alkoxylation of a monofunctional initiator such as, e.g., n-butanol or N-methyl butylamine using ethylene oxide and optionally an additional alkylene oxide such as, e.g., propylene oxide.
Copolymers of ethylene oxide with propylene oxide having more than 50% by weight of ethylene oxide are preferred, a content of 55 to 89% by weight is more preferred.
In a preferred embodiment, structural components iii) have a number-average molecular weight of at least 400 daltons, more preferred of at least 500 daltons and most preferred from 1200 to 4500 daltons.
Suitable structural components iv) include any organic compound having at least two free isocyanate groups per molecule. Examples include the diisocyanates X(NCO)2, whereby X represents a divalent aliphatic hydrocarbon radical with 4 to 12 carbon atoms, a divalent cycloaliphatic hydrocarbon radical with 6 to 15 carbon atoms, a divalent aromatic hydrocarbon radical with 6 to 15 carbon atoms or a divalent araliphatic hydrocarbon radical with 7 to 15 carbon atoms. Other examples of compounds that can be used as diisocyanate components are described, e.g., by W. Siefken in Justus Liebigs Annalen der Chemie, 562, p. 75-136.
Examples of diisocyanates that are preferably to be used are tetramethylene diisocyanate, methyl pentamethylene diisocyanate, hexamethylene diisocyanate, dodecamethylene diisocyanate, 1,4-diisocyanatocyclohexane, 1-isocyanato-3,3,5-trimethyl-5-isocyanatomethyl cyclohexane, 4,4xe2x80x2-diisocyanatodicyclohexyl methane, 4,4xe2x80x2-diisocyanatodicyclohexyl propane-(2,2), 1,4-diisocyanatobenzene, 2,4-diisocyanatotoluene, 2,6-diisocyanatotoluene, 4,4xe2x80x2-diisocyanatodiphenyl methane, 2,2xe2x80x2- and 2,4xe2x80x2-diisocyanatodiphenyl methane, p-xylylene diisocyanate, 1,3- and 1,4-diisocyanatomethyl benzene, and mixtures consisting of these compounds. 1-isocyanato-3,3,5-trimethyl-5-isocyanatomethyl cyclohexane and 4,4xe2x80x2-diisocyanatodicyclohexyl methane are particularly preferred.
It is also possible to incorporate contents of higher-functional polyisocyanates or modified polyisocyanates or polyisocyanate adducts, having, for example, carbodiimide groups, allophanate groups, isocyanurate groups, urethane groups and/or biuret groups, that are known per se in polyurethane chemistry.
Examples of structural components v) include aliphatic and/or alicyclic primary and/or secondary polyamines. Preferred examples include 1,2-ethane diamine, 1,6-hexamethylene diamine, 1-amino-3,3,5-trimethyl-5-aminomethyl cyclohexane (isophorone diamine), piperazine, 1,4-diaminocyclohexane, bis(4-aminocyclohexyl)methane, adipic acid dihydrazide or diethylene triamine as well as hydrazine or hydrazine hydrate.
Other suitable polyamines include polyether polyamines, which can be obtained by replacing the hydroxyl groups in the polyether polyols described above by amino groups. Such polyether polyamines can be produced by reacting the corresponding polyether polyols with ammonia and/or primary amines.
More preferred structural components v) include 1-amino-3,3,5-trimethyl-5-aminomethyl cyclohexane (isophorone diamine), 1,2-ethane diamine, piperazine and diethylene triamine.
The polyurethane resin dispersions according to the invention have a content of 3 to 30, preferably 7 to 17 mmol of alkali metal salts of sulfonic acids/100 g polyurethane resin solids. Such ionic groups can be incorporated by known means by the addition of structural components vi), such as diamines or polyamines containing alkali sulfonate groups, during synthesis of the polyurethane resins. Examples of suitable compounds vi) are the alkali salts of N-(2-aminoethyl)-2-aminoethane sulfonic acid. The sodium salt is preferred. The free sulfonic acids can also be incorporated during the isocyanate polyaddition process. These must then be neutralized before conversion of the polyurethane resins in water, e.g., by addition of alkali hydroxides, hydrogen carbonates or carbonates.
The present invention also relates to a process for the production of a polyurethane resin dispersion by
a) reacting in a first step the components i) to iv) to form a prepolymer, then
b) dissolving in a second step the prepolymer in an organic solvent and
c) reacting in a third step the isocyanate-containing prepolymer solution with components v) and vi),
d) precipitating, in a fourth step, the dispersion by addition of water, and
e) removing in a fifth step the organic solvent.
Free sulfonic acid groups incorporated are neutralized between the third and fourth step.
The polyurethane resin dispersions according to the invention can be produced by known methods of the prior art, such as are described, e.g., by D. Dieterich in Houben-Weyl: Methoden der Organischen Chemie, Volume E20, p. 1670-81 (1987). The polyurethane dispersions according to the invention are preferably produced in the so-called acetone process.
In the acetone process the synthesis of the aqueous preparations of polyurethane resins on which the dispersions according to the invention are based is performed in a multistage process.
In a first stage a prepolymer containing isocyanate groups is synthesized from structural components i) to iv). The dosages of individual components are calculated in such a way that an isocyanate index of 1.1 to 3.5, preferably 1.3 to 2, is obtained. The isocyanate content of the prepolymers is between 1.5 and 7.5%, preferably between 2 and 4.5%, and particularly preferably between 2.5 and 3.5%. When calculating the amounts of structural components i) to iv) it should also be ensured that the calculated number-average functionality is between 1.80 and 3.50, preferably between 1.95 and 2.25.
30 to 76.5 parts by weight of component i), 1 to 30 parts by weight, preferably 3 to 15 parts by weight of component ii), 1 to 10 parts by weight, preferably 1.5 to 6 parts by weight of component iii), 20 to 50 parts by weight of component iv), 0.5 to 13 parts by weight, preferably 1 to 5 parts by weight of component v) and 1 to 8 parts by weight, preferably 1.5 to 5.5 parts by weight of component vi) can be used, with the proviso that the sum of the components is 100.
In a second stage the prepolymer produced in stage 1 is dissolved in an organic, at least partially water-miscible solvent containing no isocyanate-reactive groups. The preferred solvent is acetone. Other solvents, such as, for example, 2-butanone, tetrahydrofuran or dioxan or mixtures of these solvents can also be used, however. The quantities of solvent to be used must be calculated in such a way that a solids content of 20 to 80 wt. %, preferably 30 to 50 wt. %, particularly preferably 35 to 45 wt. %, is obtained.
In a third stage the isocyanate-containing prepolymer solution is reacted with mixtures of the amino-functional structural components v) to vi) with chain extension to form the high-molecular polyurethane resin. The quantities of structural components are calculated in such a way that per mole of isocyanate groups in the dissolved prepolymer, 0.3 to 0.93 mole, preferably 0.5 to 0.85 mole of primary and/or secondary amino groups of structural components v) to vi) result. The calculated number-average isocyanate functionality of the resulting polyurethane resin according to the invention is between 1.55 and 3.10, preferably between 1.90 and 2.35. The calculated number-average molecular weight (Mn) is between 4500 and 250,000, preferably between 10,000 and 40,000 daltons.
In a fourth stage the high-molecular polyurethane resin is precipitated in the form of a fine-particle dispersion by addition of water to the solution.
In a fifth stage the organic solvent is partially or wholly removed by distillation, optionally under reduced pressure. The amount of water in stage four is calculated in such a way that the aqueous polyurethane resin dispersions according to the invention display a solids content of 30 to 65, preferably 35 to 55 wt. %.
The coating compositions containing the polyurethane resin dispersions according to the invention are suitable for the priming of substrates.
Suitable substrates are plastic substrates, e.g., styrene copolymers such as ASA (acrylonitrile-styrene-acrylic ester) or ASA blends, ABS (acrylonitrile-butadiene-styrene), ABS blends, such as ABS polycarbonate, polycarbonate (PC) and PC/PBTP (polybutylene terephthalate), PA (polyamide)/ABS and polyurethanes produced by the RIM (=reaction injection molding) or RRiM (=reinforced RIM) process.
The invention also relates to a paint structure containing a substrate, a primer and optionally one or more paint films, wherein the primer contains the polyurethane resin dispersions according to the invention. The invention also relates to a substrate containing a primer which is prepared from the polyurethane resin dispersion.